Embodiments of the present invention are directed to an electrostatically controlled microelectromechanical (MEM) structure. More specifically, the exemplary embodiments are directed to the control of the signal that actuates a component of the MEM structure by detecting a condition of the MEM structure as it operates.
MEM structures can come in various configurations that are suitable for use as switching devices or circuit components, such as a capacitive device.
Actuation of the MEM switch or operation as a MEM circuit component may be influenced by a control signal applied to a terminal and a beam terminal of the MEM device. The applied control signal, e.g., a “set” voltage, generates an electric field that produces an electrostatic force that causes the beam to move toward the terminal. This is similar to the concept of electrostatic force between two parallel plates. When the set voltage is applied to the terminal, the electrostatic force acting on the beam increases as the beam moves through the electric field, and closer to the terminal.
FIG. 1 illustrates the concept of electrostatic force generated by an electric field. The electrostatic force F between two parallel plates 10, 12 separated by a gap with a voltage V applied across them is given by the force equation:F=Q2÷(2×ε×A)  (Eq. 1)where Q is charge, ε is permittivity and A is the area of the plates. This electrostatic force F opposes the mechanical force of a spring S, which is trying to pull the plates apart. When the voltage V between the plates increases (V rises), the charge Q on the plates (10, 12) increases. The increase in charge Q (− and +) causes an increase of the electrostatic force F. The increased force F causes the plates (10, 12) to move closer together closing the gap, and, as a result, the capacitance C increases. If the capacitance C increases, the charge Q must increase because of the relationship Q=C×V. If the charge Q increases, the force F increases causing the gap between the plates to continue to close, and further increasing the capacitance C. This is a positive feedback loop, and when the gap is closed by, for example, ⅓, this feedback loop can become uncontrollable, and the force F increases exponentially and the top plate can collapse onto the bottom plate due to the force F.
Capacitance is also determined by the distance or, the size of the gap, between the plates (10, 12). As shown in Eq. 2, as the distance between plates of a capacitor increases, the capacitance between those plates decreases. (Eq. 2)
  C  =            ɛ      ⁢                          ⁢      A        d                  Where                    C=Capacitance in Farads            ε=Permittivity of dielectric (absolute, not relative)            A=Area of plate overlap in square meters            d=Distance between plates in meters                        
A factor during this “pull-in” effect is that the charge Q was not controllable when driven by the set voltage V. When the plates begin to close together, charge Q rushes onto the plates increasing the electrostatic force F, which can increase the closing force in a MEMs device switch. If the charge Q can be controlled, the positive feedback loop can be broken.
Accordingly, there is a need for a variable voltage to maintain better control of the charge to minimize or eliminate the “pull-in” effects of the feedback loop, and to allow the beam of the MEM device to “land” more softly, or more accurately control the movement of the beam(s) when the MEM device is actuated.